A materials separator for separating nonferromagnetic conductors from commingled nonferromagnetic materials is shown and described in U.S. Pat. No. 4,003,830, issued Jan. 18, 1977, and assigned to the present assignee. This separator is described as a ramp having embedded in its surface steady-state magnetic means in the form of strips of magnets arranged in alternating polarity at an angle to the path of commingled materials flowing down the ramp. The magnets establish a periodic series of oppositely directed static magnetic fields which induce eddy currents in conductors as they pass down the ramp. Thus, conductive materials are deflected by the magnets and thus are separated from nonconductive materials in the stream of commingled materials flowing down the ramp surface. Nonconductive materials will pass downwardly as an undeflected stream, while conductors will be deflected laterally into a separate stream to be subsequently collected as segregated conductive materials.
The inclination of the ramp may be adjusted to a desired angle to control the sliding velocity of the materials flowing down the ramp surface. However, certain limitations exist with respect to separation efficiency when the rate at which materials are fed to the ramp exceeds about 0.5 tons per hour. Since some processing plants require rates of the order of as much as 10 tons per hour, it becomes highly desirable to increase the capacity of the metal separator.
One method for achieving this is described in said U.S. Pat. No. 4,003,830 and comprises the provision of a dual ramp structure embodying a pair of juxtaposed magnetic arrays in the ramp surface.
A second method of obtaining greater capacity is to provide several basic metal separators in parallel and/or in series as described in U.S. Pat. No. 4,029,573, issued June 14, 1977, and also assigned to the present assignee, which separators may be either single or dual, as desired. It is also highly desirable to provide magnetic preseparators for removing magnetic materials from the materials being fed to the ramp-type separators.
It has been found by theoretical analysis and confirmed by experimental observation that the deflection of a metallic particle from the flowing stream of commingled particles is substantially independent of the initial velocity with which the particle enters the magnetic portion of the ramp.
The reason why the performance of the metal separator decreases with increasing feed rate is the interference between different particles. The higher the feed rate, the more likely are collisions between the particles on the ramp. These collisions tend to deflect some of the particles from the courses they would take if they did not interact with other particles. Since the frequency of collisions depends mostly upon the number of particles per unit area of the ramp surface and not upon the initial velocity, it therefore becomes advantageous to operate with a high initial velocity in order to achieve a high feed rate.
In the structure described in the aforementioned patent applications a moderately high velocity is achieved by means of a chute which guides the feed material under gravitational forces to the magnetic portion of the ramp. Attempts to increase the initial velocity by increasing the length of the chute have not proved successful in practice because air drag limits the velocity achievable by lengthening the chute.
Another disadvantage with prior art devices is that magnetic materials are often included in the commingled materials being fed to the separator ramp. These magnetic materials will adhere to and become lodged on the ramp surface and will cause other nonmagnetic particles to become undesirably scattered by collisions with the magnetic materials, thus degrading the efficiency of the separator.